Line Charts Are Not Always the Best Way to Show Time Series

I bought a personal mobile hotspot recently when I found myself in an apartment with internet that was so slow it was useless. I was counting on access to the internet to get necessary work done and to provide telephone service. The mifi unit saved my trip. I only wish their graphic monitoring my usage was as good as their product. Figure 1 shows the graphic:

The figure shows my usage from December 7 to December 14. I returned home on December 11th. They don’t have service where I live and work but that doesn’t matter since I have cable there. Therefore, I didn’t use the unit on December 12th and 13th. I had a meeting in New York on December 14th where I used the mifi again for a short time. Let’s look at the horizontal axis. It shows evenly spaced dates even though most intervals between dates represent one day while the last one represents three days. Therefore, the trend following the top of the curve is misleading.

Now look at the right end of the figure from December 11th to the 14th. It implies declining usage during this period. But I told you there was no usage on the 12th and 13th. That’s another way this chart misleads. Many blogs and books suggest line charts or area charts similar to this one for time series but they don’t work in this case since interpolation does not make sense. A bar chart showing each day’s usage would be better as shown in Figure 2. Finally, I would prefer an axis label on the vertical axis saying “Usage in GB” and not having GB repeated at every tick mark.

The takeaway is to think about your data and whether the chart you are using makes sense for your data. Don’t blindly follow rules. Times series are not always best shown using line charts or area charts.